What is the general consensus when it comes to buying used climbing gear? I've heard of people poaching cams and nuts off trad routes, but I've also heard that buying used gear from friends or online can be sketchy. Are there some guidelines that can help me cheaply fill out my gear closet?

Ropes, sure. Harnesses, sure. Even 'biners that may have been dropped... but passive pro doesn't degrade nearly as quickly. I'd think large hexcentrics, nuts, and (maybe) cams could be safe second hand... to say nothing of belay devices.

if I talk to the seller, ask for pictures, and stuff like that I'd feel comfortable buying passive pro or maybe some cams, but be prepaired if you get screwed over, inspect all the gear when you get it, and if it's suspect sucks for you. you probably won't get your money back. the good thing is most climbers arn't inherrintly bad people, they usally won't diliberately try and screw you over, be it your wallet or life. just use common sense and you should be fine.

I would never suggest that anyone buy items such as ropes, webbing or harnesses without a huge measure of trust in the seller; that is, you should probably never buy the aforementioned items (used) from anyone other than your mother, and even then you're taking a chance. I mean, she says she loves you, but she could be jivin' you too.

Passive pro is cool to buy so long as you can fondle it in your hot little hands and inspect it for frayed wires and general integrity. Cams should be inspected for frayed cables, deformity, cam integrity, sling condition, etc. Pro used for clean climbing is generally going to be in good shape, while the same pro used for aid and alpine takes a huge beating - aid pro gets bounce tested; alpine pro gets pounded into icy cracks.

Aluminum biners are generally good to go, so long as the gate pin(s) is/are in good shape, the gate opens and closes without sticking, the lock (if any) works well and the body isn't bent out of alignment in any way. Generally speaking - and despite the enormous amount of misinformation on the Intardnet regarding the topic - aluminum biners are okay even if dropped, so don't worry too much about it.

Bottom line? It's your call entirely. It's your ass that's going to someday hang from it. While I've indeed bought used gear from time to time, paying $6 for a new nut, $45 for a new screw, and even up to $100 for a new cam is still a pretty inexpensive investment in the insurance policy we call a rack.